Gas tungsten arc welding (GTAW), also called tungsten inert gas (TIG) welding, is a well known method for welding metals, in particular aluminum and magnesium. There are many known power supplies for TIG welding. Among known power supplies are phase control power supplies and inverter based power supplies. One commercially available power supply is the Miller XMT 304.TM.. U.S. patent application No. 08/584,412, now abandoned, which is hereby incorporated by reference, describes the power supply and controller used by the Miller XMT 304.TM..
Prior art power sources, when used for GTAW, are typically ac squarewave power sources. An ac squarewave power source provides a single phase output having a straight polarity (electrode negative) portion of a cycle and reverse polarity (electrode positive) portion of a cycle, with a rapid transition therebetween. It is best to have the option of selecting straight polarity, reverse polarity or alternating polarity welding from the same welding machine. Such a machine has a wider range of applications and is more desirable to the purchaser.
Some dc TIG welding applications are difficult to initiate. Specifically, it is well known in the prior art that it is difficult to start an arc when the electrode is negative. Conversely, when the electrode is positive the arc will start much easier. However, most dc GTAW is done with the electrode negative, when it is difficult to start.
Thus, one prior art GTAW inverter power supply provided an arc starting capability wherein the arc is started with a brief electrode positive polarity, even if the user selected a dc electrode negative output. This power supply is described in U.S. Pat. No. 5,444,356, which is hereby incorporated by reference. Also, when in the ac GTAW mode it provided an extended electrode positive period in order to facilitate arc starting. However, with that type of starting control, if the user selects a low output dc current with which to weld, the arc might not restart when the polarity initially switches from electrode negative to electrode positive.
A square wave output having independently adjustable duration of the two half-cycles (positive and negative) is desirable for GTAW welding. In other words, it is desirable to have the positive half cycle duration be controllable to be longer or shorter than the negative half-cycle. Hereinafter independent control of the positive and negative half-cycle widths is called adjustable balance.
AC TIG welding with adjustable balance is desirable because when the electrode is positive (with respect to the work piece) the work piece is cleaned by the arc. This is called the cleaning portion of the cycle. However, most of the energy that creates the weld is provided when the electrode is negative (with respect to the work piece). This is called penetration. Adjustable balance allows the user to select between varying amounts of penetration and cleaning as necessary for the particular welding job. It is also desirable to be able to select the frequency of the output. For example, a higher frequency output may provide a tighter arc cone and produce a better weld. Alternatively, a particular size shape or type of work piece may require operation at a lower frequency.
Prior art welding power supplies typically provide a potentiometer on the front panel of the welding power supply to select between a maximum and minimum cleaning level. The potentiometer generally selects a percentage of the complete cycle as being electrode positive, with the remainder being electrode negative, or visa versa. Often, the maximum penetration is 90 percent penetration, with 10 percent cleaning.
At a higher frequency such as 200 Hz the entire cycle is 5 milliseconds: with 90% penetration there is 4.5 milliseconds of penetration and 0.5 milliseconds of cleaning. At a lower output frequency, such as 20 Hz, the entire cycle would be 50 milliseconds, the length of penetration would be 45 milliseconds and the cleaning time would be 5 milliseconds.
Unfortunately, 5 milliseconds of cleaning will often exceed the thermal constant of a pointed tungsten tip, and cause balling of the tip. Thus, in order for the user to decrease the frequency, they would have to decrease the cleaning. In other words, at lower frequencies (longer periods) the percentage of cleaning or electrode positive must be kept short enough to prevent balling of the tungsten tip. Thus, even if the operator has selected the minimum cleaning, thinking that will protect the tungsten tip, the tip could still be ruined by creating a ball.
Accordingly, it is desirable to provide a form of balance control for the AC welding power supply that helps the user avoid balling of the tungsten tip from excess cleaning, particularly when operating at lower frequencies. Also, the power supply should provide a reliable start, particularly when a dc electrode negative at a low output current is selected.